192 University of California Publications. izoolooy. 



same time. With but one exception, small individvals reached 

 the bottom first. 



Further details may be seen by consulting the tabulation on 

 page 193. 



As has been shown, the size of tornaria varies with age. 

 The larva increases in size to a maximum larval stage and then 

 decreases with progressing metamorphosis until by the time 

 metamorphosis is well advanced the young enteropneust is much 

 smaller than the larva at its climactic period. Applying the 

 above observations to age of animals, it will be seen that the 

 rate of sinking is least for individuals of maximum larval 

 development. Individuals of the metamorphosing stages sank 

 more quickly thau the larval stages, the older ones falling 

 rapidly to the bottom of the aquarium. These observations are 

 confirmatory of the conclusions drawn from specific gravity 

 determinations. 



We are led to l-ecognize, then, that a primary purpose of the 

 swimming operations of tornaria is to enable the creature to rise 

 in the water, and to maintain itself in suspension after having 

 risen. Is this the sole value of these operations, or do they in 

 themselves accomplish something toward the horizontal distribu- 

 tion of the species? The maximum swimming rate of this tornaria, 

 as determined by many observations, is about 6 mm. per second, 

 or about seventy-one feet per hour. Over three days would be re- 

 quired for it to swim one mile. As the animal takes a spiral course 

 and at a lower average rate, the actual horizontal progress is 

 much less than these figures would indicate. The swimming 

 mechanism would seem of importance chiefly in overcoming the 

 pull of gravity, thus enabling the animal to rise and keep itself 

 in suspension within certain levels. Instead of making prog- 

 ress in horizontal directions against waves and currents, it is 

 dependent upon them for transportation. 



W. Ostwald, '02, in his "Theory of the Plankton," in which 

 he gives a careful analysis of the subject of flotation concludes 

 that viscosity of the water in which an organism lives is the 

 most important external variable factor concerned in the 

 process. He says where other factors, such as form resistance, 

 specific gravity, etc., are equal, the rate of sinking is inversely 



